An organic light-emitting diode (OLED) is a device in which organic layers are sandwiched between two electrodes and, when an electric field is applied to the organic layers, electrons and holes are injected from the electrodes and recombine in the organic layer to form excitons, and the excitons decay to the ground state and emit light. The structure of the OLED is relatively simple, requires fewer types of parts, and is advantageous for mass production. The OLEDs have been developed for display purposes, and the field of OLED lighting that uses white OLEDs has recently attracted much attention.
Unlike an OLED panel, an OLED lighting panel does not have separate red, green, and blue (RGB) pixels and emits white light using multiple organic layers. Here, the organic layers used in the OLED lighting panel include a hole injection layer, a hole transporting layer, an emission layer, an electron transporting layer, an electron injection layer, etc. according to their functions.
The OLEDs may be classified into various types depending on the materials used, light-emitting mechanisms, light-emitting directions, driving methods, etc. Here, the OLEDs may be classified according to the light-emitting structure into a bottom emission type OLED that emits light toward a glass substrate and a front emission type OLED that emits light in a direction opposite to the glass substrate. In the case of the bottom emission type OLED, a metal thin film such as aluminum, etc. is used as a cathode to serve as a reflective plate, and a transparent conductive oxide film such as ITO, etc. is used as an anode to serve as a path through which light emits. In the case of the front emission type OLED, the cathode is composed of multilayer thin films including a silver thin film, and the light is emitted through the cathode. However, the front emitting structure is rarely used as the lighting panel, except for a transparent panel that emits light from both sides, and the bottom emission structure is most widely used.
Meanwhile, a phosphorescent OLED can use all excitons, which are formed by recombination of electrons and holes, in the light emission, and thus its theoretical internal quantum efficiency approaches 100%. However, even if the internal quantum efficiency is close to 100%, about 20% of light is emitted to the outside, and about 80% of light is lost by a waveguide effect caused by a difference in refractive index between a glass substrate, an ITO layer, and an organic layer and by a total reflection effect caused by a difference in refractive index between the glass substrate and air.
The refractive index of the inner organic layer is about 1.7 to 1.8, and the refractive index of the ITO layer (i.e., a transparent electrode) generally used as the anode is about 1.9. The thickness of these two layers is as small as about 200 to 400 nm, the refractive index of the glass substrate is about 1.5, and thus a planar waveguide is naturally formed in the OLED. According to the calculation, the amount of light lost by the waveguide effect appears to be about 45%.
Moreover, the refractive index of the glass substrate is about 1.5 and the refractive index of external air is about 1.0. As a result, when light escapes from the glass substrate to the outside, the light incident beyond the critical angle causes total reflection and is trapped in the glass substrate, and the light trapped in this manner amounts to about 35%.
As a result, only about of 20% of light is emitted to the outside due to the waveguide effect between the glass substrate, the ITO layer, and the organic layer and due to the total reflection effect between the glass substrate and the air layer, and thus the external light efficiency of the OLED remains in a low level due to the low light extraction efficiency.
Therefore, a technology for extracting light trapped in the OLED is desired to improve the external light efficiency of the OLED. Here, a technology for extracting light trapped between the organic layer and the ITO layer to the outside is called internal light extraction, and a technology for extracting light trapped in the glass substrate to the outside is called external light extraction. The light extraction technologies have attracted much attention as a core technology that can improve the efficiency, brightness, and lifespan of the OLED lighting panel. In particular, the internal light extraction technology is evaluated as an effective technology that can theoretically achieve an improvement of external light efficiency of more than three times, but it sensitively affects the internal interface of the OLED. Thus, the internal light extraction technology needs to satisfy electrical, mechanical, and chemical properties in addition to the optical effect.
At present, the external light extraction technology, which attaches a micro-lens array (MLA) film, a light-scattering film, etc. to the external surface of the OLED panel, has already been established, but the internal light extraction technology has not yet reached a practical stage.
According to research reports, it is known that the internal light extraction technologies such as inner light-scattering layers, substrate surface deformation, refractive index modulation layers, photonic crystals, nanostructure formation, etc. have an effect on the internal light extraction. The key point of the internal light extraction technology is to scatter, diffract or refract the light trapped by the waveguide effect to form an incident angle smaller than the critical angle such that the light is extracted to the outside of an optical waveguide.
However, the above technologies introduced as the internal light extraction technologies are still in a laboratory stage, and thus the development of an internal light extraction technology applicable to mass production is urgently desired.